The shaft of a rotating shaft apparatus, such as a spindle of a machine tool, receives radial and thrust load. Therefore, the bearings rotationally supporting the shaft are determined in consideration of such loads. For example, Japanese Patent No. 3080253, as a first prior art, discloses a spindle which is rotatably supported by a pair of angular ball bearings and a radial ball bearing, at the front and tail end portions of the spindle, respectively. In general, a pair of angular ball bearings are disposed at the front side of the spindle. Further, Japanese Unexamined Patent Publication (Kokai) No. 7-310742, as a second prior art, discloses a configuration of a spindle which is rotatably supported by a pair of angular ball bearings and a cylindrical roller bearing, at the front and tail end portions of the spindle, respectively.
Japanese Patent No. 2677505, as a third prior art, discloses a spindle apparatus of a machine tool in which lubricating liquid is supplied into the spindle from the tail end thereof for the purpose of the cooling of the core portion of the spindle. Then, the lubricating liquid is further used for lubricating the bearings and for cooling the housing rotationally supporting the spindle. Further, Japanese Unexamined Patent Publication (Kokai) No. 6-134651, as a fourth prior art, discloses a spindle apparatus which is provided with first and second cooling units for controlling the temperature of a lubricating oil supplied into the space of a bearing, in which space the bearing balls rotate, through a central portion of the spindle, and for controlling the temperature of cooling liquid supplied into a cooling jacket defined in the housing. The first and second cooling units are controlled on the basis of the temperatures of the spindle and the housing
Recently, in the field of rotating shaft apparatuses for machine tools, increasing in the rotational speed of the shaft has been required and, therefore, the loads, on the bearings for rotationally supporting the shaft, have been also increased according to the increase in the rotational speed. Thus, DN value (the product of the inner diameter of a bearing and the rotational speed) becomes higher because the diameter of a shaft and the rotational speed thereof are increased. The higher the DN value of a shaft, the shorter the life of the bearings supporting the shaft becomes and, therefore, expensive bearings are required. Particularly, in an angular ball bearing, contacting angles are defined between the balls and the inner and outer races, which results in spinning motions of the balls and slippage of balls relative to the inner and outer races, which further results in rupturing the lubricating oil film. The centrifugal force on the balls affects on the spinning motions of the balls and, therefore, the larger the DN value, the larger the problem becomes in connection with the spinning motions of the balls of an angular ball bearing.
Further, the radial force acting on the front end portion of a shaft, to which a functional member such as a tool of a machine tool is attached, is generally larger than that acting on the tail end portion of the shaft. Therefore, there is a merit in providing the front end portion of a shaft with a diameter larger than that of the tail end portion. However, the larger the diameter of a shaft, the larger must be the diameter of the bearing. In this connection, according to the first prior art, the sizes of the angular bearings must be increased, because the spindle is rotatably supported at the front end portion by the pair of back-to-back angular bearings.
According to the second prior art, although the spindle is rotatably supported at the tail end portion by the back-to-back angular bearings, the spindle is formed with a constant diameter in the longitudinal direction, and therefore there is a problem, the same as in the first prior art, in connection with the increase in DN value accompanied with the increase in the rotational speed and the diameter of the shaft.
On the other hand, in high-speed rotating shaft apparatuses of which rotational speed is higher than 20,000 min−1, such as recent spindle apparatuses of machine tools, the internal pressure generated in the bearings for rotationally supporting the shaft must be strictly controlled. In particular, the higher the rotational speed of a shaft, the larger the outer diameters of the inner races of bearings supporting the shaft become due to the centrifugal force and the thermal expansion thereof. On the other hand, the outer diameter of the outer races is restrained by the housing. Therefore, the inner diameter of the outer race is not increased by the centrifugal force. Further, the housings of the most high speed rotating shaft apparatuses are cooled. In such a case, if the housing is too cool, excessive internal pressure is applied to the rolling elements disposed between the inner and outer races, because the outer race cannot thermally expand. Therefore, the internal race and the rolling elements are urged to the outer race so that the heat generation due to the friction between the inner race, the outer race and the rolling elements is increased, which results in damage to the bearing.
Thus, in order to prevent the excessive internal pressure on the rolling elements of a bearing, the temperatures of the spindle (the inner race) and the housing (the outer race) must be precisely controlled. In this connection, according to the third prior art, the housing is cooled by the lubricating oil, which has been heated by cooling the spindle core. Therefore, a cooling condition of the housing cannot be clearly obtained and optimal control of the temperatures of the spindle and the housing cannot be conducted. Therefore, according to the configuration of the third prior art, there is a problem that an optimal control of the temperatures of the inner and outer races of the bearings, attached to the spindle and the housing, respectively, cannot be conducted.
In this connection, according to the fourth prior art, although the first and second cooling units are controlled on the basis of the detection results from first and second temperature sensors for detecting the housing and the spindle, respectively, the first and second cooling units of the fourth prior art control the temperatures of the coolant and the lubricating oil so as to reduce the temperature gradient between the housing and the spindle. As described above, the diameter of an inner race is increased by the centrifugal force in addition to its thermal expansion. Therefore, the temperature of an inner race must be controlled in consideration of the rotational speed of the spindle. However, according to the fourth prior art, the temperatures of the inner and outer races are not measured and, therefore, the heat generated by the friction between the inner race, the outer race and the rolling elements cannot be eliminated, which may result in seizure of the bearing. Further, in the fourth prior art, the first temperature sensor detects the exterior of the housing and the second temperature sensor detects the outer surface of the front end portion of the spindle. Therefore, the temperature of the heat generating portion in the bearing cannot be precisely detected because the sensors do not detect the temperatures of the inner and outer races of the bearing.
Further, some rotating shaft apparatuses include a drive motor, that is a built-in motor disposed in the housing for rotationally driving the spindle. In such a built-in motor type rotating shaft apparatus, the housing is heated by the drive motor, in particular by its stator. Therefore, in order to cool the housing of a built-in motor type rotating shaft apparatus, the stator and a portion of the housing adjacent the stator are mainly cooled. However, in some cases, the housing is heated in the contrary by the coolant heated through the cooling of the stator. According to the third prior art, the lubricating liquid, which has been heated through the cooling of the spindle core, flows through a spiral passage adjacent the stator. However, the stator cannot be sufficiently cooled by this configuration, and the heated lubricating liquid acts as a thermal medium heating the housing.